The present invention concerns jib cranes with articulated or telescopic arms for lifting or handling loads on the grab, with a grab bucket, a grab bucket, winch or hook. It concerns in particular a device ensuring an automatic balancing of the jib of these cranes according to the variation of the length of the jib.
In cranes of this type, the jib is generally joined onto the turret and is equipped with a xe2x80x9cvariable geometryxe2x80x9d counterweight mounted on a lever arm which is deployed at the rear of the jib according to the position of the auxiliary arm of the jib. In this case, the counterweight is only used to reduce the unloading time by a fixed value without compensation depending on the length of the jib.
So as to ensure the automatic action of balancing, the document EP-A-0 033 060 concerns mechanically connecting the auxiliary arm and the counterweight by a rigid connecting rod determining a ductile parallelogram. To obtain the same ends, the document EP-A 0 110 786 concerns a system of interconnected hydraulic thrustors simultaneously activating the auxiliary arm and the counterweight.
The above-mentioned systems for balancing elements of the jig are shown to be effective, but, as they comprise variable lever arms, have the drawback in that the torque required for balancing the elements of the jig is not constant, but gradually reduces the increase of the length of the jig and hence a considerable increase of the power is required for lifting of the load. These known devices also have the drawback in that, owing to their design, the kinematics of the movements of the jig are limited. Finally, the weight of the linking connection rod (in the device of the document EP-A-0 033 060), like the weight of the thrustor of the auxiliary arm (in the device of the document EP-A-0 110 786) penalises the system.
The document U.S. Pat. No. 4,252,162 describes a loading arm for transferring products, such as petrol products, between two cisterns or tanks. This loading arm includes an internal tube borne by a support with a control thrustor for varying the position of the internal tube with respect to the support, an external tube joined to one extremity of the internal tube and a balancing girder joined to the other extremity of the internal tube. A link constituted by pulleys and cables is embodied between the external tube and the balancing girder by connecting the position of these two elements. A thrustor for controlling the external tube is borne by the internal tube and the rod of this thrustor is coupled to the balancing girder so as to control the position of the external tube.
The object of the device of the document U.S. Pat. No. 4,252, 162 is therefore not the same as the device of the invention since this concerns a liquid loading arm, that is an articulated pipework for transferring liquid between two tanks and which is thus not subjected to heavy loads and not to a crane jig whose purpose is on the other hand to lift up and move heavy loads.
In addition, according to the document U.S. Pat. No. 4,252,162, the rod of the thrustor for controlling the external tube is coupled to the balancing girder so that, during control of the position of the external tube, there is thus a variation of the lever arm which prevents having a constant torque making it necessary to use a higher power control thrustor.
The aim of the present invention is to avoid the above-mentioned drawbacks of known devices and concerns equipping the crane jig with a balancing device which, whilst being extremely effective and relatively light, eliminates the need for variable lever arms and the drawbacks they involve concerning the power required for lifting the auxiliary arm, and improves the kinematics of movements of the jig.
To this effect, the variable length crane jig of the invention is of the type including a main arm mounted pivoting on a support turret, an auxiliary arm extending the main arm by being joined to the front extremity of the latter, a counterweight secured to an articulated rocking arm fixed to the rear extremity of the main arm, and linking means between said auxiliary arm and said rocking arm of the counterweight so as to realize at any moment a total or partial balancing of the jig. It is characterised in that said linking means include a first pulley system mounted in rotation at the front extremity of the main arm and integral in rotation with the adjacent extremity of the auxiliary arm, a second pulley system mounted in rotation at the rear extremity of the main arm and integral in rotation with the adjacent extremity of the rocking arm, a flexible link connecting a point of the first pulley system to a point of the second pulley system, and an element for controlling movement of the flexible link ensuring the simultaneous pivoting of the auxiliary arm and the rocking arm so as to modify the length of the jig whilst balancing the latter, said control element being borne by the main arm and being coupled to said flexible link so as to obtain a constant lever arm by keeping constant the torque required for balancing of the elements of the jig.
Advantageously, the front extremity of the main arm is extended by two spaced parallel arms between which the rear extremity of the auxiliary arm is placed, said rear extremity being integral on each of its opposing faces with a throat pulley, the unit formed by the rear extremity of the auxiliary arm and the pulleys pivoting around a hinge pin which pivots inside transversal openings opposite said parallel arms.
Similarly, the rocking arm integral with the counterweight has towards the front two parallel arms providing between them a space for the rear extremity of the main arm, said arms each being integral with a throat pulley, and the unit formed by the arms and the throat pulleys pivot around a hinge pin which pivots in a transversal opening of the rear extremity of the main arm.
On each side of the main arm of the jig, a flexible cable connects the front pulley to the rear pulley, said cable being fixed to one point of the front pulley and to one point of the rear pulley. Thus, the movement in a direction of the two flexible cables results in the rotation of the pulleys in a given direction by provoking with a constant lever arm the lifting up of the auxiliary arm and the simultaneous tilting of the counterweight, whereas the movement of these flexible cables in the other direction results in the rotation of the four pulleys in an opposite direction, thus provoking, also with a constant lever arm, the tilting of the auxiliary arm and the simultaneous lifting up of the counterweight.
Advantageously, control of the movement of the flexible cables is ensured by a single hydraulic thrustor whose cylinder is joined onto the main arm of the jig and whose rod is integral with a transversal linking member fixed at each extremity at a point of each of the two flexible cables. The thrustor is a twin effect thrustor and is associated with a three-position hydraulic fluid distributor having a central position isolating the two chambers of the thrustor of the hydraulic pump and the fluid tank, and two side positions connecting either of the chambers of the thrustor to the hydraulic pump and the remaining chamber to the tank by selectively controlling the coming out or retraction of the rod (15) of the thrustor.